Btrfs: catch invalid free space trees

There are two separate issues that can lead to corrupted free space
trees.

1. The free space tree bitmaps had an endianness issue on big-endian
   systems which is fixed by an earlier patch in this series.
2. btrfs-progs before v4.7.3 modified filesystems without updating the
   free space tree.

To catch both of these issues at once, we need to force the free space
tree to be rebuilt. To do so, add a FREE_SPACE_TREE_VALID compat_ro bit.
If the bit isn't set, we know that it was either produced by a broken
big-endian kernel or may have been corrupted by btrfs-progs.

This also provides us with a way to add rudimentary read-write support
for the free space tree to btrfs-progs: it can just clear this bit and
have the kernel rebuild the free space tree.

Cc: stable@vger.kernel.org # 4.5+
Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com>
Tested-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: David Sterba <dsterba@suse.com>

3 files changed, 13 insertions, 1 deletions

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 33fe03551105..791e47ce9d27 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -251,7 +251,8 @@ struct btrfs_super_block {
 #define BTRFS_FEATURE_COMPAT_SAFE_CLEAR         0ULL
 
 #define BTRFS_FEATURE_COMPAT_RO_SUPP                    \
-        (BTRFS_FEATURE_COMPAT_RO_FREE_SPACE_TREE)
+        (BTRFS_FEATURE_COMPAT_RO_FREE_SPACE_TREE |      \
+         BTRFS_FEATURE_COMPAT_RO_FREE_SPACE_TREE_VALID)
 
 #define BTRFS_FEATURE_COMPAT_RO_SAFE_SET        0ULL
 #define BTRFS_FEATURE_COMPAT_RO_SAFE_CLEAR      0ULL
diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index c0bfc6ce5f06..3dede6d53bad 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -2566,6 +2566,7 @@ int open_ctree(struct super_block *sb,
         int num_backups_tried = 0;
         int backup_index = 0;
         int max_active;
+        int clear_free_space_tree = 0;
 
         tree_root = fs_info->tree_root = btrfs_alloc_root(fs_info, GFP_KERNEL);
         chunk_root = fs_info->chunk_root = btrfs_alloc_root(fs_info, GFP_KERNEL);
@@ -3131,6 +3132,14 @@ retry_root_backup:
 
         if (btrfs_test_opt(fs_info, CLEAR_CACHE) &&
             btrfs_fs_compat_ro(fs_info, FREE_SPACE_TREE)) {
+                clear_free_space_tree = 1;
+        } else if (btrfs_fs_compat_ro(fs_info, FREE_SPACE_TREE) &&
+                   !btrfs_fs_compat_ro(fs_info, FREE_SPACE_TREE_VALID)) {
+                btrfs_warn(fs_info, "free space tree is invalid");
+                clear_free_space_tree = 1;
+        }
+
+        if (clear_free_space_tree) {
                 btrfs_info(fs_info, "clearing free space tree");
                 ret = btrfs_clear_free_space_tree(fs_info);
                 if (ret) {
diff --git a/fs/btrfs/free-space-tree.c b/fs/btrfs/free-space-tree.c
index 8fd85bfbe2da..ea605ffd0e03 100644
--- a/fs/btrfs/free-space-tree.c
+++ b/fs/btrfs/free-space-tree.c
@@ -1182,6 +1182,7 @@ int btrfs_create_free_space_tree(struct btrfs_fs_info *fs_info)
         }
 
         btrfs_set_fs_compat_ro(fs_info, FREE_SPACE_TREE);
+        btrfs_set_fs_compat_ro(fs_info, FREE_SPACE_TREE_VALID);
         fs_info->creating_free_space_tree = 0;
 
         ret = btrfs_commit_transaction(trans, tree_root);
@@ -1250,6 +1251,7 @@ int btrfs_clear_free_space_tree(struct btrfs_fs_info *fs_info)
                 return PTR_ERR(trans);
 
         btrfs_clear_fs_compat_ro(fs_info, FREE_SPACE_TREE);
+        btrfs_clear_fs_compat_ro(fs_info, FREE_SPACE_TREE_VALID);
         fs_info->free_space_root = NULL;
 
         ret = clear_free_space_tree(trans, free_space_root);